Carbon sequestration is defined as a mechanism of carbon transfer from the atmosphere to the biosphere (biomass sequestration), hydrosphere (oceanic sequestration) and lithosphere (geological sequestration).

Geological Sequestration

Carbon sequestration through capture, transport and geological storage of CO2 is an important alternative in the reduction and stabilization of anthropic emissions of greenhouse gases in a sustainable perspective, based on the principle of "returning carbon back to the ground". The occurrence of natural CO2 accumulations (CO2 fields analog to gas fields) confirms the potential of geological formations to store gases for thousands or even millions of years. The McElmo Dome, in the U.S., is a natural field where CO2 is extracted for commercial use since 1983, with an original reserve of 1.6 billion tons of CO2. Since the beginning of its exploration no surface leakage has been registered.

Moreover, storage of liquids and gases in geological reservoirs is an industry ongoing activity for decades. In some countries, natural gas is injected in geological formations for later use; acid gas (with excess of sulfur) coming from industrial processes and oil refining, as well as nuclear and industrial waste are injected in geological formations for long-term storage.

The first step in carbon sequestration processes is CO2 capture from stationary sources, such as industries, refineries and coal-fired power plants.

Capture may be carried out through different processes, each one related to distinct technologies: post-combustion, pre-combustion, oxy-combustion and industrial processes.

In post-combustion and industrial processes, CO2 is extracted from flue gas using technologies such as adsorption, absorption, cryogenics or separation membranes.

In pre-combustion processes, carbon is extracted from the fuel before combustion, obtaining a gas mixture composed by carbon monoxide and hydrogen (syngas), which is further reacted with water to produce CO2. Oxy-combustion consists in burning the fuel with high levels of oxygen instead of air, obtaining almost pure CO2 as flue gas.

Following the capture process, CO2 must be compressed and transported to the appropriate geological injection site. This is accomplished mainly by pipelines or, with higher cost, by tankers.

Geological storage may be carried out safely in three types of formations: oil fields, saline aquifers and coal seams.

Oil fields are defined as an ensemble of geological reservoirs in the same region, which have been holding oil and/or gas for millions of years. The CO2 storage capacity in oil fields is estimated to be around 1000 Gt (billions of tons) (Source: IEA). CO2 injection in oil fields may result in an increased production of hydrocarbons, through a technique known as EOR (Enhanced Oil Recovery). Depending on the pressure and temperature of the reservoir, the injected CO2 will dissolve into the oil, reducing the interfacial tension and viscosity which will augment its mobility in the reservoir. This may increase the production up to 40% of the residual oil volume, which could not be recovered by conventional techniques. This technology, already employed in the U.S. since the 60's, has also been implanted in Brazil by PETROBRAS in Bahia since 1987.

Saline aquifers consist in subsurface water reservoirs with high salinity, usually similar or even higher than seawater, thus being inappropriate for consumption. Injection of CO2 in saline aquifers must be carried out in depths lower than 800 m, in order to reach the supercritical state (a gas with similar density of a liquid), which requires pressure and temperature higher than 74 bar and 31°C, respectively. These reservoirs have an enormous storage capacity, estimated in 10.000 Gt (Source: IEA).

Coal seams trap carbon dioxide in its pore spaces and the storage is carried out preferentially in deep formations, where the exploration is (and should remain) economically unfavorable. It is estimated that around 200 Gt of CO2 can be stored in coal beds worldwide (Source: IEA). As in the case of oil fields, CO2 injection in coal seams may result in production of hydrocarbons, with a technique known as ECBMR (Enhanced Coal Bed Methane Recovery). CO2 injected in the formation is preferentially adsorbed by the coal matrix, releasing the naturally-occurring methane from the coal, which can be produced with high purity. This activity has been employed commercially in the U.S. since the 80's.

One of the most important aspects of geological storage is related to the safety of this activity, which is related with the efficiency of the formation to trap the carbon dioxide, avoiding leakages to the surface or other geological strata, as well as to the risks involved with injection activities on the surface. It is estimated that the risk of CO2 injection operations is similar to that of other oil industry activities. From the geological aspect, the efficiency of formations in trapping fluids is confirmed by the natural occurrence of CO2 fields.

Since carbon dioxide is quite reactive in the injection conditions (high pressure and temperature), the adequate selection of the geological reservoir is crucial to ensure storage security. Numerical modeling techniques, validated by laboratory experiments, allow to predict with detail the behavior of injected CO2 and reservoir integrity.

Carbon dioxide injected in geological formations can be precisely monitored, measured and verified periodically using commercially available mature technologies. Among the most advanced ones is the 4D seismic, which allows to acquire subsurface images, where it is possible to verify directly the presence of CO2 injected through the rock, and measure the CO2 stored volume. Constant monitoring practices are mandatory to anticipate CO2 behavior in the subsurface, and eventually promote remediation measures.

The required technology for large-scale implantation of CO2 geological sequestration worldwide is already available. However, it still needs to be demonstrated at commercial scale. One of the main barriers is associated with the elevated costs of the capture process, which are essentially related to the high energy demands for the separation of CO2 from other flue gas, requiring up to 25% of the power plants global efficiency.

From the geological storage aspect, one the challenges is the implementation of facilities for CO2 large-scale injection in the reservoirs. Currently, the main geological storage demonstration projects inject individually around 1 Mt (millions of tons) of CO2. In this scale, thousands of injection units would be necessary to reduce significantly the carbon dioxide emissions to the atmosphere.

Among the demonstration CO2 storage projects currently in progress, StatoilHydro's Sleipner is one of the most important. Operating since 1996 in the North Sea, the Sleipner platform injects CO2, separated from natural gas, into the Utsira formation, a saline aquifer located 900 m below the North Sea bottom. It is estimated that this formation can store safely all the carbon dioxide emitted during 200 years by all the European coal-fired power plants.
The In Salah project, operated by British Petroleum in Algeria since 2004, consists in the CO2 separation from natural gas, ensuing its injection in the same geological formation, a saline aquifer 2 km below the Sahara desert surface.

The Weyburn project, a joint operation between Canada and U.S.A., is also worth mentioning. CO2 is separated from a coal power plant in North Dakota (U.S.), and transported in a 300 km pipeline to Canada, where it is injected in the Weyburn field for enhanced oil recovery (EOR) combined with CO2 geological storage.

In Brazil, CO2 injection has been carried out by PETROBRAS since 1987 in the Recôncavo Basin (Bahia) oil fields, for enhanced oil recovery. PETROBRAS, in partnership with international institutions and Brazilian universities, including CEPAC/PUCRS, is developing a series of research projects, including pilot and demonstrating CO2 geological storage projects in coal seams, oil fields and saline aquifers, in several sedimentary basins in Brazil.